The invention described herein was made by employees of the United States Government and may be used by or for the Government for governmental purposes without payment of any royalties thereon or therefor.
The invention relates to gas sensors and methods of fabrication thereof that use a silicon carbide (SiC) substrate containing an atomically flat surface so as to produce gas sensors having improved operational characteristics including stability, reproducibility, and sensitivity.
Silicon carbide (SiC) based gas sensors that have been produced thus far have inherent limitations regarding their stability, reproducibility, sensitivity, and electronic capabilities. An important factor affecting these limitations includes the starting SiC semiconductor material on which the gas sensors are fabricated. The standard SiC semiconductor has non-idealities which affect the circuits which can be produced on the semiconductor. The standard SiC semiconductor surface also has a stepped structure due to off-axis surface orientation that supports standard SiC homoepitaxial growth, typically 3-8 degrees off of the (0001) crystallographic basal plane. The sides, or risers, of the steps have a higher bond density and are more chemically reactive than the tops, or terraces, of the steps which detrimentally cause a varying surface structure on which electronic devices are fabricated. The varying surface, in turn, produces interface defects believed to degrade the performance of the gas sensors.
For gas sensors, such as Schottky diodes, capacitors, and Metal-Oxide-Silicon-Field-Effect-Transistors (MOSFETS), the surface properties of the interface strongly determine the response of the sensor. The non-ideal surface of off-axis SiC is believed to contribute to limitations in the quality of the oxides grown on SiC and thus the quality of the MOSFET devices. The presence of the many interface states in SiC has, for example, resulted in limitation in the quality of the oxides grown on SiC and thus the quality and performance of the SiC MOSFET device. From our efforts, it has been found that the reproducibility problems in Schottky diode devices created from conventional SiC wafers have hindered development in SiC-based gas sensor devices. Variations in semiconductor surface quality are likely a cause of these reproducibility problems associated with Schottky diodes. Pinning, that is where interface trap charge dominates the electrical properties of the surface of the semiconductor, often results in a less sensitive gas sensor. We believe that pinning may be related to the non-flat microscopic structure of the standard SiC surface. It is desired that a more uniform, step-free surface on which the SiC gas sensors are deposited be provided, which is believed to be a significant step forward in producing higher quality SiC based gas sensors.
It is a primary object of the present invention to provide silicon carbide (SiC) gas sensors having a SiC substrate with an atomically flat surface so as to reduce or even eliminate the performance degradations caused by poor SiC surface interface properties.
It is another object of the present invention to provide an atomically flat SiC substrate so as to produce a uniform, reproducible surface that improves the reproducibility of a gas sensing device.
It is another object of the present invention to provide gas sensors with an atomically flat substrate so as to reduce the reactivity otherwise previously normally occurring during the formation of steps on the interface between oxide layers making up the gas sensors, which contributes to the degradation of the gas sensors, and particularly, to the stability and sensitivity thereof.
Further still, it is an object of the present invention to provide gas sensors, such as Schottky diodes, capacitors, and MOSFETS, all having improved performance characteristics.
The present invention is directed to gas sensors, such as Schottky diodes, capacitors, metal-semiconductor diodes, metal-semiconductor field effect transistors, and MOSFETS and methods of fabrication thereof, all of which employ a silicon carbide substrate having an atomically flat surface, which is of primary importance to the present invention and provides for improvements in the stability, reproducibility, sensitivity, and electronic capability of the gas sensing device itself.
In one embodiment, a gas sensing device is provided that produces a change in electrical signal due to the presence of a gas, wherein the gas sensing device is comprised of a gas sensing layer residing on top of a single crystal silicon carbide (SiC) epilayer having an atomically flat surface.
In another embodiment, a gas sensing device is provided that produces a change in electrical signal due to the presence of a gas, wherein the gas sensing device is comprised of a gas sensing layer residing on top of an interface layer residing on top of a single crystal silicon carbide (SiC) epilayer having an atomically flat surface.